Get list of selected objects on specific layer
Returning only the selected objects on a layer simply requires checking the object's select property at the same time as checking which layer it is on. 

To extend the example on Get the list of objects on a layer in python:

import bpy

layerIndex = 0  #Chosen layer
objects = bpy.context.scene.objects

selectedObjects = [ob for ob in objects if ob.layers[layerIndex] and ob.select]